Highlights
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An improved formulation for flow in the downcomer shaft of a HAC is outlined.
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The role of gas solubility on the yield of gas and efficiency of HACs is explained.
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Estimates of efficiency for historical HACs are revised downward by ∼20%.
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Efficiency can be improved back to former levels by running HACs ‘hot’.
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In run-of-river HACs, gas solubility phenomena may lead to a carbon capture process